10.1002/spepro.003964 Shielding with conductive polymer and carbon fiber composites Mostafizur Rahaman, Tapan Kumar Chaki, and Dipak Khastgir Short carbon fibers mixed with blends of ethylene vinyl acetate and acrylonitrile butadiene copolymer are promising materials for shield- ing electronics from electromagnetic radiation. Materials that protect against electromagnetic interference (EMI) with high shielding effectiveness (SE) have gained great importance due to their applications in electronic, aerospace, medical, and military devices. 1, 2 Metals perform well as EMI shields, but they are relatively heavy and inflexible, with greater sensitivity to oxidation. In contrast, the prototyping of polymer conductive composites suggests that they are more chemically stable, cost-effective, and easier to process as shielding materials than their metal counterparts. 3 We investigated the development of flexible EMI shielding materi- als with high EMI SE coupled with good electrical properties such as high conductivity and low magnetic permeability. 4 Our experiments showed that the appreciable changes in DC conductivity and EMI SE occur when the fiber concentration crosses the percolation threshold, which means that the fibers begin to exhibit long-range connectivity (see Figure 1). As the short carbon fibers (SCFs) in the polymer matrix aggregate into a continuous conductive network or mesh, the compos- ite’s conductivity increases abruptly. The density of these conductive networks also governs their shielding ability: the closer the packing, the higher the EMI SE. 1, 2 In forming conductive networks with high EMI SE, polymer vis- cosity plays an important role. During mixing, SCF length reduces due to the shearing action of the molten polymer. Longer SCFs pro- duce a more conductive network. Ethylene vinyl acetate (EVA) has a lower melt viscosity than acrylonitrile butadiene copolymer (NBR) and shows higher conductivity and EMI SE than NBR at the same SCF loading. By varying the thickness of the composites, we found that the EMI SE of all samples rose linearly with the increase in thickness (see Figure 2). Greater thickness effectively means more layers of conduct- ing networks placed randomly in the path of incident electromagnetic Figure 1. (a) Direct current resistivity (P v DC) and (b) electromag- netic interference shielding effectiveness (EMI SE) versus the loading of short carbon fiber filler (F) in ethylene vinyl acetate (E) and acry- lonitrile butadiene copolymers (N). The key gives the copolymer ratios, with filler loading given per hundred parts rubber (phr). (EM) radiation. Therefore, radiation that is not intercepted by the first layer will be intercepted partly or wholly by subsequent layers. 2, 4 We also investigated the relationship between EMI SE and con- ductivity. The master curve of EMI SE versus the logarithm of Continued on next page